DFB lasers typically comprise a "grating" that serves to determine and stabilize the laser wavelength. The grating exemplarily is formed by a process that involves deposition of a conventional resist layer on the surface of a III/V semiconductor wafer, patterning of the resist layer, and etching of the exposed portions of the semiconductor surface. After removal of the resist, III/V semiconductor material of desired composition and thickness is deposited on the thus created "corrugated" semiconductor surface, and laser manufacture is completed. The grating can be disposed below or above the active layer of the laser.
Much effort has gone into devising manufacturing processes that can reproducibly provide high quality overgrowth on gratings of predetermined shape and depth, with the growth surface being smooth and plane after growth of only a few tens of nanometers. However, the results have to date not been completely satisfactory. This is especially true of ternary and quaternary III/V semiconductor materials which typically have different growth rates and compositions associated with surfaces of different crystalline orientation.
Because laser characteristics are strongly dependent on the details of the grating profile and the spacing between grating and active region, any significant wafer-to-wafer (or chip-to-chip) variation in the grating profile and/or spacing results in undesirable variations in coupling coefficiency, frequently negatively impacting device yield.
In view of the obvious advantage of being able to produce DFB lasers with high yield, it would be highly desirable to have available a manufacturing method that can repeatably provide a desired laser structure, including the grating and the layers grown thereon. This application discloses such a method.
The growth of semiconductor material on a corrugated semiconductor surface is an example of a wider processing task, namely, the growth of compound semiconductor material on a non-planar semiconductor surface. The need for such growth arises, for instance, in the manufacture of some photonic and electronic devices or integrated circuits, e.g., in the manufacture of lasers integrated with modulators.
Although the method that is disclosed herein can advantageously be used in the manufacture of a variety of InP-based semiconductor devices involving growth of compound semiconductor material on a non-planar substrate, the method will be described below in tens of a specific and important embodiment, namely, the manufacture of InP-based DFB lasers involving the growth of ternary or quaternary III/V semiconductor material such as GaInAsP on a corrugated Inp surface, or the growth of InP on a corrugated ternary or quaternary III/V semiconductor (e.g., GaInAsP) surface.
By an "InP-based" device we mean a III/V semiconductor device formed by epitaxial growth on an InP substrate, or containing one or more layers of InP.